For Fox News, there is one big question: What does life after Bill O’Reilly look like?

The abrupt and once-unthinkable defenestration of Mr. O’Reilly on Wednesday leaves Fox News without the top-rated host in cable news — and with a potentially far-reaching shake-up in the channel’s vaunted evening lineup.

Tucker Carlson, a conservative provocateur who joined Fox News’s prime-time ranks only three months ago, has been tapped to replace Mr. O’Reilly at 8 p.m. Eastern, beginning on Monday, the network said. Mr. Carlson has become, seemingly overnight, one of the network’s most vital players, a remarkable turnaround for a pundit whose bow-tied heyday had seemed behind him.

Fox News’s afternoon ensemble show, “The Five,” which features a chatty round table of hosts, will take over Mr. Carlson’s 9 p.m. slot.

If the exit of Mr. O’Reilly were not enough to rattle the executive suites at Fox News, the network must now navigate a drastic on-air overhaul at a rare moment when the channel is facing a challenge from its rivals.

Fox News’s prime-time dominance over the last 15 years is a mantra inside the network. But last month, Rachel Maddow of MSNBC surged past Mr. Carlson at 9 p.m. in the coveted 25-to-54-year-old age bracket. CNN just announced a prime-time test next week for Jake Tapper, whose sharply critical analysis of President Trump has made him a journalistic star of the moment.

The prospect of Ms. Maddow and Mr. Tapper at 9 p.m. would amount to a pincers move by Fox News’s rivals to dethrone their longtime Nielsen ratings dominator.

The new prime-time lineup will also test whether Fox News viewers are loyal to particular anchors or to the network at large. When Megyn Kelly left the network in January, her audience did not; Mr. Carlson has had higher ratings at 9 p.m. than did Ms. Kelly, who left for NBC.

But Mr. O’Reilly is another matter. He has cultivated a huge fan base that reliably tuned into his show for more than a decade, making him not only a TV star but also a best-selling author.

Network executives are hoping that those viewers will not be alienated by his dismissal.

“Fox News has demonstrated again and again the strength of its talent bench,” the head of the network, Rupert Murdoch, said in a memo to staff members on Wednesday. “We have full confidence that the network will continue to be a powerhouse in cable news.”

This will be Mr. Carlson’s third time-slot change in four months — an unheard-of fluidity at Fox News after years of a more-or-less static lineup. Since January, Mr. Carlson has been the replacement for three departing Fox stars: Greta Van Susteren, Ms. Kelly and, now, Mr. O’Reilly.

The only remaining member of Fox’s longtime four-hour stretch of nighttime programming is Sean Hannity, who draws huge ratings and will continue to command his 10 p.m. slot.

Mr. O’Reilly’s ratings primacy was no small matter. Before a New York Times article on April 1 disclosed settlements involving sexual harassment claims against him, he averaged nearly four million viewers, more than 700,000 viewers better than Mr. Carlson’s second-place 9 p.m. show. Mr. O’Reilly also gave a kick start to Fox News’s enormous ratings edge from 8 p.m. to 11 p.m.: The channel averages more than 2.8 million viewers in prime time, nearly twice as many as its rival MSNBC. CNN, which has more viewers than MSNBC among those 25 to 54 years old, averages about 1.1 million viewers.

Even before Mr. O’Reilly’s departure was announced, senior Fox News executives were scrambling to lock down a revamped slate of evening anchors, with Mr. Murdoch leading the discussions, according to a person told of the network’s deliberations who spoke on the condition of anonymity to discuss internal talks. The network was eager to piece together a lineup that could be up and running by Monday, when Mr. O’Reilly had been scheduled to return from a vacation in Italy.

Inside Fox’s subterranean Manhattan newsroom, uncertainty about the channel’s on-air lineup was matched by speculation about the fate of its behind-the-scenes players.

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Mr. O’Reilly forged an often controversial on-air persona over his two-decade career at Fox. He was forced out on Wednesday in the wake of several sexual harassment allegations.CreditCreditAndrew Hetherington/Redux Pictures

Bill Shine, the network’s co-president, who had been a right-hand man to the former Fox News chairman Roger E. Ailes, was a vigorous advocate for retaining Mr. O’Reilly during conversations over the past two weeks. Mr. Shine remains closely associated with Mr. Ailes’s reign, but some people at Fox News say he is likely to stay onboard as a sign of stability during a time of unprecedented change at the network. Mr. Ailes was forced out last summer after numerous sexual harassment claims were made against him. Both Mr. O’Reilly and Mr. Ailes have denied the accusations against them.

As for Fox News, it looked within for its revamped programming roster.

The channel said it would give Eric Bolling, who has been a substitute anchor for Mr. O’Reilly and a host of “The Five,” his own afternoon program at 5 p.m. beginning on May 1.

In its new prime-time incarnation, “The Five” will add a regular panelist: Jesse Watters, a protégé of Mr. O’Reilly’s who recently interviewed Mr. Trump. Mr. Watters is best known for prankster-style antics and was heavily criticized last year for a segment that trafficked in offensive stereotypes about Asian-Americans.

Rounding out the lineup of “The Five” will be Kimberly Guilfoyle, Dana Perino, Bob Beckel, Greg Gutfeld and Juan Williams. The show will be broadcast live, as will “Tucker Carlson Tonight,” which gives Fox News back-to-back hours of live cable news in prime time. (Mr. O’Reilly, like Mr. Hannity, often taped his show.)

Fox News will keep Martha MacCallum in the 7 p.m. slot, which she was awarded on a temporary basis in January. Her show, now a permanent part of the schedule, will be called “The Story With Martha MacCallum” and will also be live.

A version of this article appears in print on , on Page B1 of the New York edition with the headline: For Fox News, Life After Bill O’Reilly Will Feature Tucker Carlson. Order Reprints | Today’s Paper | Subscribe